In many chemical processes pumps are used to transfer liquids. Many of these pumps employ mechanical seals to avoid leakage around the pump shaft. These seals usually consist of a rotating and a stationary element having sealing faces which are perpendicular to the pump shaft and in sliding contact. The faces are polished, lubricated parts held together under a pressure sufficient to prevent escape of the liquid being pumped.
The mechanical seals are usually contacted with a sealing liquid, i.e., a pump seal flush. This flush serves the purpose of lubricating and cooling the sealing faces and also helps prevent the leakage of air or liquid around the pump shaft. In many pumps the seal flush is the same liquid being moved by the pump; in other pumps a seal flush is supplied from an external source and can be a different liquid.
When a pump is being used to transfer a liquid slurry, problems can be created if the slurry is used as the seal flush. The solids present in the slurry will often cause a stoppage in the seal flush line, thus preventing flow. Also, if the solids are hard or abrasive, they can shorten the useful life of the sealing faces of the seal.